An evergreen is a tree or plant that doesn't shed its leaves in the winter, but stays green all year. A Christmas tree is an evergreen.
Even in the middle of winter, when so many trees in cold climates have bare branches, evergreens brighten up the landscape with vibrant green needles or leaves. Fir trees, holly bushes, and eucalyptus shrubs are all examples of evergreens. You can describe all of them using evergreen as an adjective as well: "I made the wreath using evergreen branches."